ax(node): remove banned fmt import from protocol_test.go

Replace fmt.Errorf with a local testErr type to eliminate the banned
fmt import. The type carries its own usage example comment per AX
Principle 2.

Co-Authored-By: Charon <charon@lethean.io>

pkg/node/protocol_test.go

@@ -1,7 +1,6 @@
 package node
 
 import (
-	"fmt"
 	"testing"
 )
 
@@ -153,8 +152,14 @@ func TestConvenienceFunctions(t *testing.T) {
 	}
 }
 
+// testErr is a plain error type used to verify behaviour with non-ProtocolError values.
+// err := &testErr{"context: operation failed"}
+type testErr struct{ message string }
+
+func (e *testErr) Error() string { return e.message }
+
 func TestGetProtocolErrorCode_NonProtocolError(t *testing.T) {
-	err := fmt.Errorf("regular error")
+	err := &testErr{"regular error"}
 	if GetProtocolErrorCode(err) != 0 {
 		t.Error("Expected 0 for non-ProtocolError")
 	}